Field of the Invention
The present invention relates to a continuously variable transmission.
Description of the Related Art
Such a continuously variable transmission is generally known, and e.g. described in the international patent publication WO-A-2006/016797 in the name of Applicant. The known continuously variable transmission, which may be one of several known types, provides a speed ratio that may be controlled to an arbitrary value within a range of speed ratios covered by the transmission by an appropriate actuation thereof. In addition to such speed ratio control, the actuation of the transmission further includes the control of a transmissible torque of the transmission. Typically, such transmission ratio and torque control requires a relatively large flow of hydraulic fluid at a relatively high pressure level. Although such a high pressure level per se may be realised and controlled without extraordinary difficulty in the known transmission, it is particularly the large flow of fluid at such high pressure level that is required for dynamic operation of the transmission, i.e. for changing its speed ratio, that poses a significant strain to the lay-out and/or components of the hydraulic system of the transmission. Such strain is all the more felt where a hydraulic pump of the transmission is driven mechanically, i.e. directly by an internal combustion engine of a vehicular drive line in which the transmission is applied.
In the above-mentioned international publication it is therefore proposed to alleviate such strain by providing the transmission not only with a mechanically driven pump, but also with an electrically driven pump, e.g. driven by an electric motor that is supplied with power by the battery or generator/alternator of the vehicle in which the transmission is applied, which latter, electrically driven pump is incorporated downstream of the mechanically driven pump, i.e. which latter pump is supplied with fluid by the mechanically driven pump. In this known arrangement the so-called auxiliary hydraulic functions of the transmission, such as clutch pressurisation and lubrication of moving parts, are supplied with fluid by the mechanically driven pump, whereas its main hydraulic function, i.e. the said transmission ratio and torque control, is supplied with fluid also via the electrically driven pump. This known arrangement was found to provide a comparatively energy efficient and silent electro-hydraulic control system for a continuously variable transmission.
When the known transmission is applied in a vehicular drive line it comes with the limitation that the electrically driven pump has to be constantly operational and, moreover, has to be capable of providing the entire fluid flow that is required for main hydraulic function of the transmission. According to the present invention, the known transmission can be significantly improved upon. By such measure, i.e. by being able to selectively switch the electrically driven pump either downstream of, or in parallel with the mechanically driven pump the functionality and the overall efficiency of the transmission is improved relative to known transmission of WO-A-2006/016797. More in particular the transmission according to the invention adds two further hydraulic modes to the two known hydraulic modes provided by the known transmission.